The present invention relates to an automatic Y-axis offset system for a cathode ray tube display, which permits the position of a trace on the display to be immediately and automatically returned to a position on the tube face at which the trace is visible, when a signal having a large DC component has been applied to the display system.
It is frequently necessary to examine signals which contain a relatively high DC component, using a cathode ray tube display such as that of an oscilloscope. When such a signal is first applied to the cathode ray tube display system, then, assuming that DC coupling and amplification are provided between the input signal terminals and the deflection plates of the cathode ray tube, the voltage applied to the Y-plates of the cathode ray tube will frequently be of such a magnitude that the display trace is completely off the cathode ray tube face, i.e. is not visible. With a conventional cathode ray tube display system, it is then necessary for the operator to manually adjust the Y-axis level adjustment potentiometer to bring the trace to a position at which it becomes visible. This is a time-cosuming and troublesome task. If the adjustment sensitivity of the Y-axis level adjustment is made relatively low, so that a relatively small movement of the display trace occurs in response to movement of the Y-axis level adjustment control by a certain amount, then it is possible to adjust the position of the trace in a convenient and precise manner. However, in this case, bringing the trace to a visible position on the cathode ray tube screen, when it has been displaced off the screen by a large input signal voltage as described above, becomes excessively time-consuming. If, on the other hand, the Y-axis level adjustment sensitivity is made relatively high, so that a small amount of movement of the adjustment control results in a relatively large displacement of the trace on the display tube face, then although the trace can be fairly rapidly brought to a position of visibility when it has disappeared off the tube face due to the application of a high input signal voltage, subsequent adjustment of the position of the trace cannot be performed in a precise and convenient manner.
There is therefore a requirement for a system which will automatically position the trace on the cathode ray tube face at a position where it becomes visible, irrespective of the DC voltage level of the input signal, before examination of that signal is begun. This requirement is met by the present invention, which performs such initial positioning of the trace in an automatic and rapid manner.